The present invention relates to a heating system employing both a heat pump and a source of supplemental heat, such as a resistance heat furnace, and in particular to a control system whereby most efficient utilization of the two heat sources is realized.
A heat pump utilizes a compressor and a refrigerant recirculation system including a condenser and evaporator to provide both cooling in the warmer seasons of the year and heating in the winter. The dual function of the heat pump is accomplished by locating the low temperature evaporator in the interior space during the summer, and the high temperature condenser is located in the interior space during the winter months.
Since heat pumps utilize ambient air as the heat source when operating with the condenser in the space to be heated during the winter months, they operate efficiently only when the outside air temperature is above a certain level, such as 40.degree.. In regions of colder average winter temperatures, supplemental heat, such as is supplied by fossil fuel furnaces or resistive heat, is necessary in order to maintain the temperature within the building at the desired level. Furthermore, as the outside ambient temperature drops below 40.degree., the efficiency of the heat pump suffers because of frost buildup on the evaporator coils In the defrost cycle, the heat pump is run in the reverse direction to supply heat to the outside evaporator coils thereby melting the frost, following which normal operation be resumed. Of course, during the defrost cycle of the heat pump, heat is not being supplied to the building, and supplemental heat must be relied on to maintain the desired ambient temperature. Accordingly, supplemental heat is required when the heat pump is in the defrost cycle and when the outdoor temperature is below that which permits adequate transfer of heat from the outdoor ambient air to the interior of the building.
Whenever the outside ambient temperature is below that which permits adequate transfer of heat, both the heat pump and the supplemental heat source are operating simultaneously, with greater energy demand than with the heat pump operating alone or with the supplemental heat operating alone. When the heat pump and supplemental heat source are operating together beyond a certain portion of a heat cycle, there is greater energy consumption than if only the supplemental heat source alone is used for a given quantity of heat delivered. During the defrost cycle of the heat pump, energy is required to heat the outside evaporator coils, and supplemental heat may be necessary to maintain the desired inside temperature level. Accordingly, if frequent and lengthy defrost cycles are necessary to maintain the evaporator coils free of frost, less energy will be consumed by operating the supplemental heating alone and shutting down the heat pump entirely. This is true even though heat pump operation is generally more efficient than supplemental heating, for example resistance or fossil fuel burning, depending on the outside temperature and humidity conditions.
For example, consider a heating system using a 15 kilowatt heat pump and a 30 kilowatt resistive heater. The heat cycle is one hour, that is, for any given period of one hour, the interior zone to be heated will demand heat input. The most efficient case exists when heat is required solely from the heat pump, because maximum energy consumed is 15 kilowatt-hours. However, should conditions require that the heat pump and resistive heater be run in tandem, the maximum energy consumption is 45 kilowatt-hours, which is substantially less efficient than if only the resistive heater ran for the cycle consuming 30 kilowatt-hours. Weather conditions can permit either the heat pump operating alone or operating in tandem with the resistive heater during the heat cycle.
Prior art control of heat pump operation is generally accomplished by means of an electromechanical thermostat mechanism, with separate temperature sensors for each state of heat pump system operation. Furthermore, there are defrost timers, relays, and pressure and temperature sensors utilized to control system defrost cycling, but since they are internal to the heat pump system, they are not readily available for immediate user control. Furthermore, the weather and condition of the heat pump system form a complex set of factors that are constantly changing, thereby making it very complex to determine the combination of heat pump and resistive heating which renders maximum efficiency. To maintain the heat pump system in the most efficient state, would require the user to continually measure all of these factors and perform complex computations. Accordingly, prior art heat pump installations do not have the means available to the user to effectively operate the heat pump systems in the lowest energy demand state while maintaining the temperature of the building at the desired level.
To summarize, the current problem with heat pump installations is that their advantage over other methods of heating exists only when the heat pump operates without supplemental heat. The more frequently that the heat pump operates with supplemental heat, either during its heating cycle or defrost cycle, the less advantage there is in terms of energy efficiency over other heating plants, such as resistance or fossil fuel furnaces. This has resulted in heat pumps being used more often in regions where the outdoor temperatures are sufficiently high during the winter months that the need for supplemental heat is infrequent, such as in the southern and southwestern regions of North America. The use of heat pumps in cooler northern climates, particularly those climates where the air humidity is high during the winter months, requires very complex controls which, although they can perhaps be justified for large buildings, are not feasible for domestic and smaller commercial and industrial installations.